Toxicity and Repellency of (E/Z)-3-Butylidenephthalide: A Natural Compound Isolated from Ligusticum porteri Root Extract Evaluated Against Imported Fire Ants (Hymenoptera: Formicidae)

Imported fire ants are pests of significant importance, especially in the southern United States. We tested (<i>E</i>/<i>Z</i>)-3-butylidenephthalide, a natural compound that was isolated from the ethanolic extract of <i>Ligusticum porteri</i> roots, as a repellent and toxicant against workers of imported fire ants. A series of serial concentrations, starting from 156 µg/g until failure, were tested using digging bioassays. Workers removed significantly less sand from the vials treated with (<i>E</i>/<i>Z</i>)-3-butylidenephthalide as compared to the ethanol control. Based on sand removal data, (<i>E</i>/<i>Z</i>)-3-butylidenephthalide treatment resulted in a more significant digging suppression against red imported fire ant workers at concentrations ranging between 19.5 and 0.6 µg/g than the solvent control whereas sand removal at 0.3 µg/g was similar with the solvent control. Black imported fire ants showed repellency at serial concentrations ranging between 19.5 and 0.15 µg/g whereas the hybrid imported fire ants showed repellency between 19.5 and 4.9 µg/g. In DEET treatments, red and black imported fire ants showed repellency at dosages of 125 to 62.5 µg/g, whereas the treatment failed at the dose of 15.6 µg/g in hybrid fire ants. (<i>E</i>/<i>Z</i>)-3-butylidenephthalide with LC₅₀ values of 11 and 16.4 µg/g was toxic against red and black imported fire ants, respectively, followed by hybrid imported fire ants (LC₅₀ = 104.7 µg/g). Fipronil with LC₅₀ values of 0.49, 0.33, and 0.53 µg/g against red, black, and hybrid fire ants, respectively, was more toxic than (<i>E</i>/<i>Z</i>)-3-butylidenephthalide. In residual bioassay, toxic activity lasted for 3 weeks at dosages of 250 and 500 μg/g against HIFA. The high repellency and toxicity of (<i>E</i>/<i>Z</i>)-3-butylidenephthalide against imported fire ants makes it a natural compound of interest for further evaluation under natural field conditions.